1. Field of the Invention
This invention relates to a method for sizing of particles of a water-absorbent resin having a surface cross-linked layer, and more particularly to a method for producing a water-absorbent resin which is destitute of fine powder and coarse particles.
2. Description of the Related Art
The water-absorbent resin possesses a characteristic property of absorbing a large volume of water and is used as materials for forming such sanitary articles as disposable diapers, sanitary napkins, and incontinence pads, water retaining agent for soil, and drip sheets for foodstuffs. Particularly, for use as such sanitary articles as disposal diapers, a water-absorbent resin manifesting a high absorption capacity under pressure is sought preferably owing to reduce the thickness.
When such a water-absorbent resin having a particle size exceeding 850 μm is used for a disposable diaper, it emits a coarse touch and degrades the sensation to the user's skin. The resin is deprived of such coarse particles by classification before it is manufactured into the product. Further, it is generally held that the water-absorbent resin is preferred to have as small a content of particulates of not more than 150 μm as permissible. The fine powder degrades the ability to pass liquid in the products and causes a leakage of the liquid in the use of the products.
As a means to obtain a water-absorbent resin having a small content of fine power, the method which classifies a water-absorbent resin prior to subjecting it to surface cross-linkage and removes the fine powder in advance has been known. The method is removing the fine powder not larger than 150 μm by classification, mixing the residual with a surface cross-linking agent and thereafter heating them, thereby obtaining a water-absorbent resin having a surface cross-linked layer. In spite of the preliminary removal of the fine powder, the end product still contains fine powder.
As a means to produce a water-absorbent resin having a small content of fine powder and excelling the absorption capacity under pressure, a method which comprises a step of adding a dried powder of water-absorbent resin and a cross-linking agent together, and a step of pulverizing the resultant mixture during cross-linking the surface region of particles while using coarse particles of water-absorbent resinmeasuring 600-1000 μm has been known (JP-A-1999-302391). This method is supposed to obtain an absorbent having a small content of fine powder and manifesting a high absorption capacity under pressure and a satisfactory ability to pass liquid under pressure by using a water-absorbent resin of a large particle diameter instead of a water-absorbent resin of a small particle diameter which is liable to form glob of swollen gel, and cross-linking the surface region of the resin during pulverizing at least part of the resin particles. Mixing devices which are usable for effecting the surface cross-linkage and the pulverization include a cylindrical mixing device, a double-wall conical mixing device, a V-shaped mixing device, a ribbon type mixing device, a meat chopper, and a rotary mixing device, for example.
One known method effects surface cross-linkage of a water-absorbent resin by using a horizontal stirring and drying device, a rotary drying device, a disk drying device, a fluidized-bed drying device, a ventilation drying device, or an infrared ray drying device at a temperature in the range of 90-250° C. (JP-A-1992-214734). As a means to perform a covalent bond type surface cross-linkage on a dried finely pulverized hydrated polymer, a method which comprises injecting an aqueous solution containing a surface cross-linking agent into a Lödige type mixing device and mixing it over a period of 5-10 minutes, then heating the resultant mixture, and cooling to afford a product is described (Example 7, JP-A-2002-527547). In this example, however, since the product was classified to a particles size traction of 120-850 μm, an aggregate was formed.
Another known method comprises preparing an internal cross-linked polymer having particle diameters adjusted by screening in the range of 150-850 μm, adding this polymer to a surface cross-linking agent, and heating the resultant mixture by the use of a paddle mixer to effect surface cross-linkage (The official gazette of International Unexamined Patent Publication JP-2002-515079). For this example sieving of a product was carried out at 850 μm and, consequently, an aggregate was formed.
The conventional methods for producing the water-absorbent resins were that even when a disintegrating step and a classifying step had been incorporated prior to surface cross-linkage, the obtained surface cross-linked water-absorbent resin contained coarse particles and that when the resin had been deprived of such coarse particles by classification, the separated coarse particles were discarded as useless. The present invention is directed at providing a method for producing a water-absorbent resin destitute of fine powder and coarse particles while sparingly inducing formation of fine powder from disintegrating.